It has been a problem that concrete structures in sewage-treating facilities become gypsiferous and deteriorated. This deterioration is known to be ascribable to sulfur-oxidizing bacteria of the genus Thiobacillus which oxidize hydrogen sulfide to sulfuric acid. These bacteria usually live widely in soil or water and grows by oxidation of sulfur compounds and assimilation of carbon dioxide. The sulfur-oxidizing bacteria of the genus Thiobacillus are known to exist not only in the portion of structures contacting sewage, but also in structures in the sewage-treating facility facing to the air but not contacting sewage, depending on the species thereof.
Various methods have been proposed for preventing such deterioration of the structures. Most of practically performed methods include coating the structures with anti-corrosive materials. However, the coating of the anti-corrosive material can easily be damaged from pinholes or scratches, and thus has problem in durability. In addition, coating on a joint of small-diameter tubes is difficult. Further, this method is not advantageous in terms of cost.
Then, another method is proposed in Japanese Laid-open Patent Application No.4-149053, in which deterioration of the structural materials due to the sulfur-oxidizing bacteria of the genus Thiobacillus is prevented by directly admixing to concrete a particular metal or a particular metal oxide insoluble in water but soluble in sulfuric acid, such as nickel, instead of coating the surface of the structures with the anti-corrosive materials. In this method, nickel completely prevents sulfur oxidation activity, respiration, and carbon dioxide gas-fixing activity of the sulfur-oxidizing bacteria when pH value is around neutrality. This method is excellent for sufficiently preventing deterioration due to the sulfur-oxidizing bacteria.
However, for practically inhibiting the growth of the sulfur-oxidizing bacteria to prevent deterioration of the concrete by adding nickel to the concrete, about 0.1 parts by weight of nickel is being added to 100 parts by weight of cement. It is therefore desired to develop an inhibitor that exhibits an excellent effect in inhibiting growth of sulfur-oxidizing bacteria in smaller adding amount.
It is known that molybdenum, ammonium molybdate or a mixture of ammonium molybdate and tungsten activates growth of Thiobacillus novellus, whereas tungsten, when used alone, inhibits growth of the same bacteria (Journal of Bacteriology, Vol.153, No.2 (1983) William M. et al. "Sulfite Oxidase Activity in Thiobacillus novellus" p.941-944). On the other hand, it is also reported that, although molybdenum (Mo.sup.4+) activates growth of Thiobacillus novellus as noted above, it inhibits growth of Thiobacillus thiooxidans (Chemical Abstracts, Vol. 95, No.1 (Jul. 6, 1981) p127 (1081a)). Thus, even the sulfur-oxidizing bacteria of the same genus Thiobacillus have different growth inhibitory mechanism. Therefore, even if a certain growth inhibitor is recognized to have an effect on sulfur-oxidizing bacteria existing in neutral pH range of 6 to 8 such as Thiobacillus novellus, such an inhibitor is not used in acidic pH range of 2 to 6, in which Thiobacillus novellus scarcely exists. Further, it is not believed that a substance which inhibits growth of the aforementioned Thiobacillus novellus equally inhibits growth of sulfur-oxidizing bacteria of the genus Thiobacillus existing in pH range of 2 to 6 such as Thiobacillus thiooxidans.
Therefore, growth inhibitors proposed hitherto are not always effective to all of the sulfur-oxidizing bacteria of the genus Thiobacillus.